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Bureau Seminar, August 31, 2012

Process of Sedimentation & Depositional Architecture of a Deepwater Local Base-of-Slope Channel & Levee-Overbank Apron:
The Upper Miocene Upper Mount Messenger Formation, Taranaki Basin, New Zealand

Jonathan R. Rotzien
Geological & Environmental Sciences, Stanford University

Deepwater base-of-slope settings record sediment delivery at the transition from the continental slope to basin floor and represent an important environment characterized by both incision and deposition. The structure and complex set of transport and depositional processes operating in base-of-slope environments, in particular, remain understudied. We revisit the Upper Miocene Upper Mount Messenger Formation of the Taranaki Basin, North Island, New Zealand for two reasons other than it offers extensive exposure (nearly 6 km) along Pukearuhe Beach: 1) it provides a unique opportunity to evaluate strata from a local base-of-slope setting using an outcrop perspective and 2) though often cited and well studied, the Upper Mount Messenger Formation lacks a process-based sedimentology study. Analysis of stratal architecture, facies distributions, paleotransport direction indicators and stratigraphic sections from the base to top of the formation show the following: (i) avulsion splay aprons deposited in an overbank setting are concentrated in the lower and middle parts of the formation and are composed of at least five laterally continuous, fining upward packages. These packages compose roughly 80% of the exposures on Pukearuhe Beach and consist of basal aprons of thick- to medium-bedded planar and ripple laminated turbidites with moderate amounts of scour and small-scale, low-relief channelization that grade and fine upward into very thin-bedded, structured, mud-rich and heavily burrowed turbidites. The sand-rich channelized basal portions do not incise the underlying mud-rich package caps. (ii) Updip to the upper part of the formation, these packages are incised by two major erosional surfaces that cut from the top of the outcrop to the base in a southerly direction, interpreted as northern limb channel margins, that are infilled by burrowed mudstone and onlapped by thick- to very thick-bedded, structured sandstone and mudstone. Sandstone beds within these channels decrease in thickness and percentage of planar laminated divisions above the margin and increase in proportion of ripple laminated divisions. (iii) The two channel margin packages are separated by a thick ~40 m portion of stratigraphy composed of predominantly climbing-ripple cross-laminated medium-bedded sandstone and mudstone that is interpreted as a sandy levee to the channels. Intriguingly, throughout the outcrop, the southern limbs of channel margins are not exposed. In addition, the onlapping relationships, paleocurrents, evidence for lateral accretion and the uniform offlapping trajectory of the channel fill indicate the progressive stepping and lateral migration of deposits within the north-trending channels to the southwest. The results highlight the processes of sedimentation that are dominated by low density turbidity current deposition, stratal architecture and overall evolution of a sand-rich, channel and levee-overbank depositional environment in a deepwater, local base-of-slope setting. In this case, we envisage a feeder system with a narrow, canyon-incised shelf where waves and/or wind played an important role in winnowing and refining the ultimate sand-rich sediment delivered to the deepwater. Much of the stratigraphy and time is preserved in the overbank environment, whereas in the channelized environment, time is missing due to the frequent erosion due to scouring and incision.


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